Systems for contacting solids and particulate material are well known and routinely employed in the processing of gases, the production of chemicals, and the refining of petroleum. The particulate materials in most cases comprise catalysts or adsorbents and the process streams are gaseous or liquid mixtures of reactants, product or streams undergoing separation.
One particularly well known method of contacting particulate material with a fluid stream retains the particulate solid material as a bed of particulate material through which the fluid stream passes. Long known advances in the methods and apparatus for contacting particulate materials with gaseous streams have employed means for moving the particulate material while the processing of the gaseous fluid is underway. A particular form of this system moves the particulate material through a contacting zone in plug flow. Individual particles rest on each other to make up the bed of particles as opposed to fluidized processes in which an upward flow of gaseous material lifts the particles to permit fluidized transport of particles through the bed of solids. Systems for intermittently or continually moving particles in a plug flow bed greatly advanced the art of reactant and particle contacting by eliminating the need to shut down process equipment in order to change out particles after they have ceased to function due to deactivation or saturation.
A multitude of arrangements with various bed geometries are known for contacting the particulate material with the fluid streams. Such arrangements include radial flow beds where particulate solids are retained in an annular ring or downflow or upflow beds where fluid streams pass through a cylindrical bed or laminar bed of particulate solids. This invention is directed to an arrangement wherein the particulate solids are retained in relatively narrow vertically extended flow channels through which the particulate solids and fluids move in a cocurrent direction. This arrangement is formed by using thin plate members to define flow channels or catalyst retaining tubes having the catalyst on the inside of the tubes. The use of plates or tubes to define flow channels are particularly important in applications that require or benefit from heating or cooling of the particulate solids and fluids within the flow channels to control the temperature of a reaction or other processing. In such arrangements, the thin plates or tubes provide a large area of heat transfer surface by which a heat transfer fluid may indirectly contact one surface of the tube or plate while the other surface retains the particulate solids and fluids. For example, concurrent indirect heat transfer in which a reactant fluid contacts a catalyst and reaction fluid can be used to supply or withdraw the heat of reaction in an endothermic or exothermic process to establish isothermal conditions in the reaction zone.
While such systems controlling temperature are known, simultaneous movement of particulate solids during the reaction and heat transfer requires the recovery of the particulate material from a large number of flow channels or tubes within a contacting vessel. Simultaneous recovery of the fluid stream from the flow channels must occur with the recovery of particulate material. The simplest method for collecting the particulate material and for recovering the fluid stream is to discharge them into an open volume or chamber at the bottom of the flow channels while also discharging the fluid stream to the same chamber of collection area.
It has been found that under some conditions the discharge of the fluid stream along with the particles into a common collection chamber can cause a phenomena generally referred to as "blow out" where the fluid stream causes fluidization at the outlet of the flow channel. The occurrence of blow out is related to the mass flux of the fluid stream passing through the particulate material in the conduit. As the mass flux increases, it increases a frictional drag force on the particulate material which increases the pressure tending to discharge the particulate material from the flow channel. Thus the mass flux of fluid through the particulate material can be maintained until a critical pressure gradient is reached at which point all of the particulate material is rapidly discharged from the conduit. The resulting relatively empty conduit then provides an unrestricted flow path for a large volume of the fluid stream to by-pass the particulate material in the other conduits. Blow out may also occur when the vertically upward flow of fluid locally around the discharge end of the conduit reaches a velocity where it fluidizes the solids in which the outlet of a flow channel is buried. Once the particles around the outlet reach fluidization, particles flow freely away from the flow channel outlet and remove all resistance to the downward pressure on the particles within the flow channel, and again, the flow channel rapidly empties.
There are known methods for preventing blow out which include providing a restriction at the bottom of the flow channel outlets. While such devices will raise the pressure gradient required from the fluid stream before blow out is reached, blow out will still eventually occur with sufficient mass flux and gradient through the conduit. Thus consideration of blow out will still impose a limitation on gas flux through the flow channels.
It is known from U.S. Pat. No. 4,975,036 to disengage gaseous fluids from particular material upstream of the outlet end of a conduit from which particulate material is discharged. This type of system incorporates a cylindrical screen section into a cylindrical conduit for transporting catalyst particles. The screen section is located well upstream of the conduit outlet and are sized to prevent passage of the catalyst particles through the screen members while permitting the escape of gases therethrough. The systems shown herein transfer the catalyst particles through extended conduits in dense phase and collect the particles in a confined conduit arrangement. Confinement of the particles prevents blow out from occurring but limits the number or complicates the arrangement of the flow channels.
Moreover, the radial discharge of fluid streams from conduits carrying particulate material are subject to another phenomena generally referred to as pinning. As the gas velocity increases, it reaches a point where the frictional forces generated by the gas passing across the particulate solids overcome the weight of gravity acting downward on the solids and hold the particulate material against the screen thereby restricting the flow of particulate material downwardly through the flow channel or conduit. In relatively small diameter conduits, those having a diameter of six inches or less, even small mounts of pining can quickly block all movement of particulate material down the flow channel.